The present invention relates to a novel process for the catalytic preparation of 3-isochromanones from phthalans.
3-Isochromanones and their substituted derivatives are of great interest, for example, as intermediates in the synthesis of pharmaceuticals and crop protection agents. Thus, EP-A-0 278 595 and WO 97/12864 describe the use of 3-isochromanone as an intermediate in the synthesis of fungicides and pesticides.
3-Isochromanones are prepared from phthalans by the process of Azzena et al. (Tetrahedron Lett. 1995, Vol. 36, page 8123ff), Almena et al. (Tetrahedron 1995, Vol. 51, p. 3351ff) and Churanov et al. (Vestn. Mosk. Univ., Khim 1975, Vol. 16, 338 ff.). To this end, the corresponding 1,3-dihydroisobenzofurans (phthalans) are reacted with lithium and catalytic amounts of naphthalene to give the dilithium compound of the formula below ##STR3## which is subsequently converted into the desired 3-isochromanone by reaction with carbon dioxide.
This process requires two equivalents of metallic lithium per equivalent of product.
The consequence of this high consumption of lithium is that transfer of this process to industrial scales is uneconomical, in particular owing to the safety precautions associated with metallic lithium and especially owing to the high costs.